A critical parameter of a high performance infrared tracking system of the FLIR type is uniformity in brightness of the displayed video. The term used for non-uniformity is commonly called shading. Sources of shading in a common module FLIR system are vidicon camera, video CRT display unit and a fundamental physical characteristic of optical systems called the cosine to the fourth power effect. Another effect which is unique to each system design is a thermal gradient across the aperture stop in front of the FLIR scanner.
Typical values of shading for vidicon camera shading is 15 to 25 percent on good quality vidicons. Ten to 15 percent shading for display systems is typical. For a plus or minus 6.5 degree scan angle, the cosine to the fourth power is 0.975 which yields a center to edge difference of 2.5 percent. For example, for a condition where the scene to system aperture temperature difference is 40 degrees centigrade and the maximum system intensity transfer is 1.5 degrees centigrade, a shading of 67 percent results. The magnitude of cosine effect alone exceeds a typical shading requirement by a factor of two to three.
Shading correction circuits using digital techniques can be used. However, these approaches are relatively complex and require extensive hardware. The FLIR dynamic shading compensation circuit of the present invention is an analog approach and requires minimum hardware which interfaces with the existing common module electronics.